The character of the ground and optically excited states was investigated by quantum chemical calculations. We propose a rung ground state with V $3d_{xy}^1\mathrm{\text{−}}\mathrm{O}$ $2p_y^1\mathrm{\text{−}}\mathrm{V}$ $3d_{xy}^1$ character, instead of the conventional picture of one unpaired electron shared by 2 V ions. The unpaired electron on O is low-spin coupled to the V $d$ electrons and spin density is predicted to be localized on vanadium. The absorption peak at 0.9 eV is assigned to a state with similar orbital occupations but a different spin coupling scheme, resulting in spin density localized on the bridging oxygen.

• Received 15 February 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.076407